Heat oil in a nonstick skillet over medium heat.,Add onion; cook 10 minutes, stirring frequently.,Add bell pepper and black pepper; cook 3 minutes or until bell pepper is crisp-tender.,______,Top each slice with 2 ounces beef, 1/4 cup onion mixture, and 1 cheese slice.,Place sandwiches on a baking sheet coated with cooking spray; broil 2 minutes or until cheese melts.,Note: These are open-faced sandwiches, but you can top with bread slices to make them a 'standard' sandwich.